vl_imbackwardmx.c: fixes a bug arising in some special cases
vl_imbackwardmx.c: fixes a bug arising in some special cases

j-bo/vlfeat,rxl194/vlfeat,cvfish/vlfeat,qubick/vlfeat,richarddzh/vlfeat,rxl194/vlfeat,CansenJIANG/vlfeat,CansenJIANG/vlfeat,hongzhili/vlfeat,erexhepa/vlfeat,qubick/vlfeat,qubick/vlfeat,vlfeat/vlfeat,rxl194/vlfeat,cvfish/vlfeat,hongzhili/vlfeat,richarddzh/vlfeat,mylxiaoyi/vlfeat,mylxiaoyi/vlfeat,WangDequan/vlfeat,richarddzh/vlfeat,erexhepa/vlfeat,erexhepa/vlfeat,cvfish/vlfeat,vlfeat/vlfeat,erexhepa/vlfeat,qubick/vlfeat,richarddzh/vlfeat,cvfish/vlfeat,CansenJIANG/vlfeat,j-bo/vlfeat,CansenJIANG/vlfeat,hongzhili/vlfeat,vlfeat/vlfeat,mylxiaoyi/vlfeat,cvfish/vlfeat,mylxiaoyi/vlfeat,vlfeat/vlfeat,WangDequan/vlfeat,mylxiaoyi/vlfeat,mylxiaoyi/vlfeat,cvfish/vlfeat,j-bo/vlfeat,WangDequan/vlfeat,j-bo/vlfeat,erexhepa/vlfeat,richarddzh/vlfeat,CansenJIANG/vlfeat,WangDequan/vlfeat,vlfeat/vlfeat,qubick/vlfeat,rxl194/vlfeat,erexhepa/vlfeat,hongzhili/vlfeat,vlfeat/vlfeat,j-bo/vlfeat,WangDequan/vlfeat,WangDequan/vlfeat,CansenJIANG/vlfeat,hongzhili/vlfeat,hongzhili/vlfeat,richarddzh/vlfeat,rxl194/vlfeat,qubick/vlfeat,rxl194/vlfeat